In analog-to-digital converters, such as delta-sigma A/D converters, the continuous-time analog input must be sampled. Non-linear charge injection from the sampling switches limit the linearity of the converter. Various methods have been used in the past to improve the linearity of prior art sampling networks in order to provide analog-to-digital converters having signal to distortion ratios of 100-110dB. However, additional linearity improvements are required to go beyond 110 dB.
In order to avoid high frequency noise, it is a general practice in industry to provide an anti-alias filter in the continuous-time to discrete-time interface of the sampling circuit. This filter for over-sampled systems is usually a passive RC network. Such filters are often shown in data sheets for high precision A/D converters. For example, the Crystal Semiconductor Corporation Data Book, Volume 1, A/D Conversion IC's, April 1992, shows on pages 2-164 and 2-182 single ended RC filters, and on pages 2-198 and 2-216 differential input single ended filters. The Crystal data sheet for the CS5389, dated February, 1993, shows on page 6 a differential input differential filter. Another example is the Analog Devices data sheet for the AD1879, dated May 30, 1991, which shows differential input differential filters in FIGS. 17, 18, and 24a.
The charging current required by the sampling network must be supplied by the filter. This current develops a voltage drop across the input resistor of the filter. The sampled waveform will be distorted if the current has a non-linear dependency on the analog input.
One method used in the past to overcome this distortion is to place a unity gain amplifier inside the integrated circuit at the input voltage terminal. The output of the unity gain amplifier to used to rough charge the sampling capacitor and then the input voltage terminal is connected to the sampling capacitor directly to fine charge the capacitor. Such a circuit is shown in FIG. 2 on page 6-111 of the aforementioned Crystal data book. However the unity amplifier must have its gain and operation characteristics carefully controlled, and it is very difficult to provide a unity gain amplifier which can provide outputs which are close to the Vss or Vdd supply voltages. At these extreme voltage swings the distortion is worst. When the unity gain amplifier is needed the most, when the input is near one of the supply voltages, the unity gain amplifier provides little benefit.
Therefore it will be appreciated that a sampling circuit which reduces the nonlinear effects of charge injection for a wide range of input signals is highly desirable.